Yogini Ekadashi occurs during the waning phase of the moon in the Hindu month of Ashadha (June – July) as per traditional Hindu calendar followed in North India. In 2015, the date of Yogini Ekadashi is June 12. The importance of this Ekadasi was explained to Yudhisthira by Lord Krishna. Fasting on this Ekadashi helps in overcoming the remorse caused by sins committed and also helps in achieving ‘moksha.’
The significance of Yogini Ekadasi is mentioned in Brahma Vaivarta Purana. The Purana narrates the tale of a gardener named Hemamali who overcame a curse, which made him a leper, by observing a fast on Yogini Ekadasi. The gardener was instructed by sage Markandeya about the Ekadashi.
A fast on this day is believed to help in washing away the sins committed and in leading a pious life.
All the usual rules associated with Ekadasi fasting are observed on the day. The fast begins with the sunrise on Ekadasi day and ends with sunrise on the next day.
source – http://www.hindu-blog.com/2008/06/yogini-ekadasi.html